Put activity heats up as ISRG is targeted by a short seller

Put traders have set their sights on Intuitive Surgical, Inc. (NASDAQ:ISRG - 500.97) in recent sessions, according to data from the major options exchanges. During the past five days, speculators on the International Securities Exchange (ISE) and Chicago Board Options Exchange (CBOE) have bought to open 3,008 puts on ISRG, compared to only 1,148 calls. The stock's five day ISE/CBOE put/call volume ratio of 2.62 reveals that bearish bets have nearly tripled their bullish counterparts over the past week.

Broadening our view, ISRG has racked up a 10-day put/call volume ratio of 1.80 on the ISE, CBOE, and NASDAQ OMX PHLX (PHLX). This ratio arrives in the 100th percentile of its annual range, suggesting options players have been scooping up puts over calls at their fastest pace of the past year.

In the same pessimistic vein, the Schaeffer's put/call open interest ratio (SOIR) for ISRG stands at 1.19, with puts outnumbering calls among options set to expire within three months. This ratio arrives in the 88th percentile of its annual range, as short-term speculators have been more put-heavy only 12% of the time during the last year.

ISRG has been free-falling on the charts this week, with traders apparently responding to a negative report from short seller Citron Research. The stock is down nearly 3% at last check, adding to Wednesday's 5.7% slide. Now, the shares are testing tenuous support at the round $500 level, which previously contained lows for ISRG in early June.